The present invention relates to a method for generating synchronization bursts for OFDM transmission systems, a method for synchronizing wireless OFDM systems, an OFDM transmitter as well as to a mobile communications device comprising such a transmitter.
The present invention relates generally to the technical field of synchronizing wireless OFDM (orthogonal frequency division multiplexing) systems. Thereby it is known to use a synchronization burst constructed using especially designed OFDM symbols and time domain repetitions.
Particularly from the document IEEE P802.11a/d2.0 xe2x80x9cDraft supplement to a standard for telecommunications and information exchange between systemsxe2x80x94LAN/MAN specific requirementsxe2x80x94part 1: wireless medium access control (MAC) and physical layer (PHY) specifications: high-speed physical layer in the 5 GHz bandxe2x80x9d a synchronization scheme for OFDM systems is proposed. This document is herewith included by reference as far as it concerns the synchronization including the proposed implementation. Said known scheme will now be explained with reference to FIG. 6 to 8 of the enclosed drawings.
FIG. 6 shows the structure of the known synchronization field. As shown in FIG. 6 the synchronization field consists of so-called short symbols t1, t2, . . . t6 and two long symbols T1, T2. In view of the present invention particularly the short symbols t1, t2 . . . t6 are of interest. Among the short symbols t1, t2, . . . t6 used for the amplifier gain control (t1, t2, t3) and the course frequency offset and timing control only the symbols t1, t2, t3 and t4 are actually generated, whereas the symbols t5, t6 are cyclic extensions (copies of the symbols t1 and t2, respectively). It is to be noted that FIG. 5 shows only the synchronization preamble structure as the structure of the following signal field indicating the type of baseband modulation and the coding rate as well as the structure of further following data fields are not of interest in view of the present invention. For further details reference is made to said prior art document.
The symbols t1, t2, t3, t4 are generated by means of an OFDM modulation using selected subcarriers from the entire available subcarriers. The symbols used for the OFDM modulation as well as the mapping to the selected subcarriers will now be explained with reference to FIG. 6.
Each of the short OFDM symbols t1, . . . t6 is generated by using 12 modulated subcarriers phase-modulated by the elements of the symbol alphabet:
S={overscore (2)}(xc2x11xc2x1j)
The full sequence used for the OFDM modulation can be written as follows:
Sxe2x88x9224,24=2*{1+j,0,0,0,1+j,0,0,0,xe2x88x921xe2x88x92j,0,0,0,xe2x88x921xe2x88x92j,0,0,0,1xe2x88x92j,0,0,0,xe2x88x921xe2x88x92j,0,0,0,0 0,0,0,1+j,0,0,0,1+j,0,0,0,xe2x88x921xe2x88x92j,0,0,0,1+j,0,0,0,xe2x88x921+j,0,0,0,1+j}
The multiplication by a factor of 2 is in order to normalize the average power of the resulting OFDM symbol.
The signal can be written as:             r      SHORT        ⁢          xe2x80x83        ⁢          (      t      )        =            w      SHORT1        ⁢          xe2x80x83        ⁢          (      t      )        ⁢          xe2x80x83        ⁢                  ∑                  k          =                                    -                              N                2                                      /            2                                                N            s                    /          2                    ⁢              xe2x80x83            ⁢                        S          k                ⁢                  xe2x80x83                ⁢        exp        ⁢                  xe2x80x83                ⁢                  (                      j            ⁢                          xe2x80x83                        ⁢            2            ⁢                          xe2x80x83                        ⁢            π            ⁢                          xe2x80x83                        ⁢            k            ⁢                          xe2x80x83                        ⁢                          Δ              F                        ⁢                          xe2x80x83                        ⁢            t                    )                    
The fact that only spectral lines of Sxe2x88x9224, 24 with indices which are a multiple of 4 have nonzero amplitude results in a periodicity of TFFT/4=0.8 xcexcsec. The interval TTSHORT1 is equal to nine 0.8 xcexcsec periods, i.e. 7.2 xcexcsec.
Applying a 64-point IFFT to the vector S, where the remaining 15 values are set to zero, four short training symbols t1, t2, t3, t4 (in the time domain) can be generated. The IFFT output is cyclically extended to result in 6 short symbols t1, t2, t3, . . . t6. The mapping scheme is depicted in FIG. 7. The so called virtual subcarriers are left unmodulated.
The way to implement the inverse Fourier transform is by an IFFT (Inverse Fast Fourier Transform) algorithm. If, for example, a 64 point IFFT is used, the coefficients 1 to 24 are mapped to same numbered IFFT inputs, while the coefficients xe2x88x9224 to xe2x88x921 are copied into IFFT inputs 40 to 63. The rest of the inputs, 25 to 39 and the 0 (DC) input, are set to zero. This mapping is illustrated in FIG. 7. After performing an IFFT the output is cyclically extended to the desired length.
With the proposed inverse fast Fourier transform (IFFT) mapping as shown in FIG. 7 the resulting time domain signal consists of 4 periodically repeated short symbols t1, t2, t3, t4, and cyclically extended by a copy of t1, t2, which copy is depicted in FIG. 5 as t5, t6. Note that in the present case only spectral lines with indices which are a multiple of 4 have nonzero amplitude. Other periodic natures can be generated by setting other multiples of the spectral lines to nonzero amplitudes.
Though the known synchronization scheme is very effective, it provides for disadvantage regarding the time domain signal properties.
For OFDM (or in general multicarrier signals) the signal envelope fluctuation (named Peak-to-Average-Power-Ratio=PAPR) is of great concern. A large PAPR results in poor transmission (due to nonlinear distortion effects of the power amplifier) and other signal limiting components in the transmission system (e.g. limited dynamic range of the AD converter).
For synchronization sequences it is even more desirable to have signals with a low PAPR in order to accelerate the receiver AGC (automatic gain control) locking and adjusting the reference signal value for the A/D converter (the whole dynamic range of the incoming signal should be covered by the A/D converter resolution without any overflow/underflow).
FIGS. 8a, 8b show the xe2x80x9cabsolutexe2x80x9d (sqrt{In*+Quad *Quad}) value of the resulting time domain signal waveform with the sequences proposed by Lucent Technologies. Oversampling (8*) was considered in order to ensure the peak was captured correctly using the limited 64-point IFFT.
FIGS. 8c, 8d show the real and imaginary part of the resulting transmitted time domain waveform. The resulting PAPR is 2.9991 dB (no oversampling) and 3.0093 dB (with 8 times oversampling).
Therefore it is the object of the present invention to provide for a synchronization technique which bases on the known synchronization technique but which presents improved time domain signal properties to reduce the requirements for the hardware.
The above object is achieved by means of the features of the independent claims. The dependent claims develop further the central idea of the present invention.
According to the present invention therefore a method for generating synchronization bursts for OFDM transmission systems is provided. Symbols of a predefined symbol sequence are mapped according to a predefined mapping scheme on subcarriers of the OFDM system wherein the symbols of the predefined symbol sequence represent subcarriers with nonzero amplitudes. A synchronization burst is generated by inverse fast Fourier transforming the subcarriers mapped with a predefined symbol sequence. According to the present invention the predefined symbol sequence is optimized such that the envelope fluctuation of the time domain signal (Peak-to-average-power-ratio) is minimized.
The predefined symbol sequence can be chosen such that the following equations are satisfied for all symbols of the predefined symbol sequence:
n=2m,
Cixe2x88x921=xc2x1C1xe2x88x92i,
n being the number of symbols of the predefined symbol sequence,
m being an integer larger than one,
C being the symbol value, and
i being an integer running from 1 to m.
The mapping of the symbols of the predefined symbol sequence and the Inverse Fast Fourier Transform can be set such that the resulting time domain signal of the synchronization burst represents a periodic nature.
Alternatively the mapping of the symbols of the predefined symbol sequence and the Inverse Fast Fourier Transform is set such that one burst part of the synchronization burst in the time domain is generated and the periodic nature of the synchronization burst in the time domain is achieved by copying the one burst part.
The number of symbols of a symbol sequence (n) can for example be 12.
The above equations define generally the symbol sequences according to the present invention. The predefined symbol sequence can therefore be for example:
A A A xe2x88x92A xe2x88x92A xe2x88x92A xe2x88x92A A xe2x88x92A xe2x88x92A A xe2x88x92A,
wherein A is a complex value.
Alternatively the predefined symbol sequence can be:
A xe2x88x92A A A xe2x88x92A A A A A xe2x88x92A xe2x88x92A xe2x88x92A,
wherein A is a complex value.
Alternatively the following predefined symbol sequence can be used:
A B xe2x88x92A B xe2x88x92A xe2x88x92B B A xe2x88x92B A xe2x88x92B xe2x88x92A,
wherein A, B are complex values.
As a further alternative the following sequence can be used:
A xe2x88x92B xe2x88x92A xe2x88x92B xe2x88x92A B xe2x88x92B A B A B xe2x88x92A,
wherein A, B are complex values.
According to the present invention furthermore a method for synchronizing wireless OFDM systems is provided, wherein a synchronization burst is generated according to a method as set forth above and the synchronization burst is transmitted respectively before the transmission of data fields.
Thereby the time domain signals of the synchronization burst can be precomputed and stored in a memory, such that the computation of the time domain signal of the burst is only effected once.
According to the present invention furthermore a OFDM transmitter is provided comprising a mapping unit for mapping the symbols of a predefined symbols sequence according to a predefined mapping scheme on subcarriers of the OFDM system, wherein the symbols of a predefined symbols sequence represent the subcarriers of the OFDM system with nonzero amplitudes. Furthermore an inverse fast Fourier transforming unit is provided for generating a synchronization burst by inverse fast Fourier transforming the subcarriers of the OFDM mapped with said predefined symbols sequence. The mapping unit thereby is designed such that the resulting time domain signal of the synchronization burst represents a periodic nature. The mapping unit according to the present invention uses a predefined symbol sequence which is such that the envelope fluctuation of the time domain signal of the synchronization burst is minimized.
According to the present invention furthermore a mobile communications device such as set forth above is used.